> > > >

1948-49 Theatre Catalog, 7th Edition, Page 403 (390)

1948-49 Theatre Catalog, 7th Edition
1948-49 Theatre Catalog
1948-49 Theatre Catalog, 7th Edition, Page 403
Page 403

1948-49 Theatre Catalog, 7th Edition, Page 403

Light Generation by the High-Intensity Carbon Are

Theoretical Considerations Assembled in the Course of Arc-Carbon Research and Development

In the motion picture industry, light is perhaps the most important single factor in the recording and reproducing processes involved. Light is thrown in carefully controlled quantities and distribution patterns on the motion picture sets and on the actors. Reflected portions of this incident energy are directed toward a camera lens, and focused to produce a permanent record of this refiected pattern on film. Finally, light is selectively absorbed by prints from this film in a theatre, so that the distribution pattern reaching the original film in the studio camera may be recreated on the theatre screen. Light is also an essential agent in the recording and reproduction of sound, although that important phase of the industry will not be included in this present consideration.

' The high-intensity carbon arc is such a commonplace and generally useful light source in the photographic and projection processes which characterize the motion picture industryt' 3 that little thought ordinarily is given to the physical processes involved in the operation of such a source. In the belief that cone cepts found useful in the laboratory in directing the development of new and brighter carbons may be of interest to the ultimate users of such carbons, the present discussion has been prepared.

To begin with, consider the simple arc circuit of Fig. 1. Here a direct-current source of perhaps 110 volts is connected through a series resistor to a pair of carbons. In common with all gaseous discharges, the carbon arc has a negative resistance coefficient: as the current


Research Laboratories, National Carbon Company, Inc.. Cleveland, Ohio

SummaryeThe theory of light production in the high-intensity carbon arc is discussed, together with a description of the phenomena associated with the initial striking of the arc and the maintenance of the electric discharge through the arc stream. The formation of the positive carbon crater is described and the factors defined which determine the maximum current loading which a particular carbon electrode will support. The importance of efficient heat dissipation from the positive crater region in extending the useful current range of a given-sized carbon is pointed out, and the efectiveness of water cooling in providing better heat dissipation at this point is noted.

is increased, the ohmic resistance of the arc becomes less. Some ballast effect, such as is provided by the series resistor in this case, must, therefore, be incorporated in the circuit. To start the arc, the two carbon electrodes are brought into brief contact, drawn apart again, and a light source of very high intensity is produced where nothing but air existed before.

FIG. 1. A typical carbon-arc circuit.

When the power is first applied, nothing happens, because the circuit includes an air gap between the carbon electrodes which cannot be broken down by the relatively low voltage of the power source. It is not until the electrodes are brought into physical contact that current starts to fiow. In a series circuit such as is established when the electrodes touch, the same current fiows throughout, so that the relative heat in any portion of the circuit is determined by the resistance of that portion. At the point of contact between the electrodes, the cross-sectional area is small, so that the resistance is high. As a result, a high concentration of heat is produced at this point; and as the pressure on the electrodes is reduced, preparatory to separating them, the contact area grows smaller and smaller, so that it continues to become hotter and hotter.

In order to explain what happens next, it is necessary to consider an atomic property of hot bodies called thermionic emission. The atoms in any solid substance are in a continual state of vibration, with electrons revolving rapidly around each nucleus in a variety of orbits at various distances. When the substance is heated, the atomic vibrations grow more intense, and the electrons spin faster and faster through wider and wider orbits. In the case of those atoms next the surface, an occasional electron will break away altogether, and as the heating continues more and more electrons fiy off into space.

Hot carbon is not so good an emitter of electrons as the materials used for this purpose in vacuum tubes, but it does possess this ,property to an appreciable degree. Thus, to return to the arc, as the last pair of atoms is about to be drawn apart in the separation of the electrodes, the concentration of current and the intensity of the resultant heat are terrific, sufficient not only to cause thermionic emission, but to vaporize the carbon itself at the tiny area of final contact. Consider also what would happen at the instant of separation if no arc were to form, so that the current would fall abruptly to zero. The full open-circuit line voltage would immediately appear across the gap, and if the initial gap is assumed to be a millionth of an inch, and the line voltage 100 volts, a voltage gradient of 100,000,000 volts per inch would be established promptly. As a matter of fact, the distance between atoms in solid carbon is something of the order of 20 billionths

Reprint o] a paper presented April 22, 1947. at I'll Socier of Motion Picture Engineer, Convention in Chicago and published in the Journal 0/ the S. Ml'. 13.. September. 1947.

1948-49 Theatre Catalog, 7th Edition, Page 403